Commercial vehicles often include pumping equipment that is driven by a power source provided by the vehicle or a power source mounted on the vehicle. A common example is a tank truck including loading and unloading equipment for transporting and delivering a solid product, such as grain, plastic pellets or powder cement, or a liquid product, such as liquid waste, chemicals or fuel oil. Tank truck loading and unloading equipment typically includes a blower, a compressor or a vacuum pump driven by a power take-off (PTO) from the truck engine, an electric motor or an auxiliary gasoline or diesel engine. Historically, the preferred method of powering tank truck pumping equipment has been a mechanical driveline coupled to the PTO from the truck engine, and the amount of space on the truck chassis occupied by the mechanical driveline has not been problematic. As a result of recently enacted environmental regulations lowering the permissible amount of diesel emissions and pollutants, the space on a tank truck chassis occupied by a compliant exhaust system severely limits the amount of space available for installation and operation of pumping equipment driven by a mechanical driveline.
The use of the PTO from the truck engine and a mechanical driveline to drive pumping equipment on a tank truck has further shortcomings in addition to space limitations. Proper operation of the pumping equipment for a particular application requires the selection of a blower, compressor or vacuum pump having a suitable speed, pressure and/or vacuum. Driving a blower, compressor or vacuum pump via a PTO and mechanical driveline, however, results in the speed of the pumping equipment being directly proportional to the speed of the truck engine. Similarly, when the pumping equipment is directly driven by an electric motor via a coupling and/or gearbox, the speed of the blower, compressor or vacuum pump is directly proportional to the speed of the electric motor. Operation of the pumping equipment under its minimum rated operating speed can result in damage to the blower, compressor or vacuum pump. Damage to the pumping equipment can also result from operating the blower, compressor or vacuum pump beyond its maximum rated operating pressure or vacuum. The pressure and vacuum relieving devices, such as mechanically or electrically actuated valves, currently available require frequent and repeated maintenance since they exhibit a tendency to seize as a result of corrosion resulting from environmental conditions and road debris.
Many of the deficiencies associated with the use of a truck engine PTO and mechanical driveline can be overcome by using a hydraulic drive system to drive pumping equipment on a tank truck. With a hydraulic drive system there is no direct connection to the motive power provided by the PTO from the truck engine. Instead, a hydraulic pump connected to the PTO pumps hydraulic fluid (e.g. oil) to a hydraulic motor, which in turn drives the blower, compressor or vacuum pump. As a result, the hydraulic pressure produced by the hydraulic pump is directly proportional to the horsepower requirements of the blower, compressor or vacuum pump. Proper sizing of the hydraulic components (i.e. hydraulic pump and hydraulic motor) will cause the hydraulic drive system to initially operate the blower, compressor or vacuum pump at the correct speed, pressure and/or vacuum. Other advantages of a hydraulic drive system include the ability to place the blower, compressor or vacuum pump at any convenient location on the tank truck. In addition, hydraulically driving a blower, compressor or vacuum pump eliminates the potentially dangerous rotating shafts that are typically exposed with a mechanical driveline coupled to a truck engine PTO, as well as the accompanying alignment concerns. Further benefits of a hydraulic drive system relative to a mechanical drive line include slower start-up and emergency shutdown capability.
As the components of the hydraulic system wear over time, however, the hydraulic pump will lose efficiency and provide less oil to the hydraulic motor for the same operating speed. In turn, the hydraulic motor will generate less power and cause the blower, compressor or vacuum pump to operate at a slower speed. As previously mentioned, operation of the pumping equipment below its minimum rated operating speed can result in damage to the blower, compressor or vacuum pump. Increasing wear also causes a rise in the oil temperature. In the event the oil temperature exceeds the cooling capacity of the hydraulic heat exchanger (commonly referred to as the hydraulic cooler), the oil will begin to breakdown and consequently cause damage to the components of the hydraulic drive system, particularly the pump and motor. Damage to the hydraulic components will further reduce the efficiency of the hydraulic pump and the power of the hydraulic motor, thereby further slowing the operating speed of the blower, compressor or vacuum pump and causing additional damage when the pumping equipment is operated below its minimum rated operating speed. If the available pressure or vacuum relief valve does not function suitably, the blower, compressor or vacuum pump may operate above (or below) the design pressure causing the hydraulic oil pressure to rise and generate additional heat. As previously mentioned, the hydraulic drive system will overheat and damage the hydraulic components if the oil temperature exceeds the cooling capacity of the hydraulic cooler.
As previously mentioned, a significant advantage of a hydraulic drive system as opposed to a mechanical driveline or electric motor is the ability to control the speed of the pumping equipment. However, many truck engines require the operator to select the correct engine speed (i.e. RPM) for the hydraulic drive system. In the event the operator incorrectly selects an excessive engine speed, the hydraulic pump likewise operates at excessive speed and consequently increases the amount of hydraulic oil flowing to the hydraulic motor, which in turn increases the operating speed of the blower, compressor or vacuum pump. The increased speed allows the blower, compressor or vacuum pump to absorb additional horsepower. The additional horsepower absorbed by the blower, compressor or vacuum pump increases the pressure of the hydraulic oil and adds additional heat to the hydraulic drive system. As previously described, if the additional heat exceeds the capacity of the hydraulic cooler, the hydraulic drive system will overheat and damage the pumping equipment.
Regardless, an operator can overheat a hydraulic drive system and potentially damage the blower, compressor or vacuum pump as a result of over-pressurization. The hydraulic oil temperature will rise in all instances in which the hydraulic drive system is operating improperly and, if the hydraulic oil temperature exceeds the capacity of the hydraulic cooler, the components of the hydraulic drive system will be damaged, potentially also damaging the pumping equipment. It is possible to provide the blower, compressor or vacuum pump with flow-compensating or temperature-compensating controls to maintain the speed of the pumping equipment relatively constant. Such controls, however, are costly, require frequent maintenance and inherently susceptible to failure. Likewise, over-pressurization and the resultant over-heating can be avoided with suitable electrical sensors and flow control equipment, such as a hydraulic oil temperature transducer and a valve actuator that operates in response to the temperature transducer to vent the blower/compressor pressure or vacuum pump vacuum. Again, however, such sensors and control equipment adds significant cost, complexity and reliability concerns.
Accordingly, there exists an unresolved need for an apparatus and method for the protection of a hydraulic drive system and pumping equipment driven by the hydraulic drive system. More particularly, there exists a need for a protection valve for a hydraulic drive system and a method for protecting a hydraulic drive system and pumping equipment driven by the hydraulic drive system. There exists a specific need for a purely mechanical protection valve including means for venting air pressure or vacuum to the ambient atmosphere in response to a hydraulic oil temperature that exceeds the capacity of the hydraulic cooler of a hydraulic drive system.